fastfmt

cargo fmt, then put the one-liners back.

Stable rustfmt expands constructs such as short function bodies, statement-position if/else, and short match or struct bodies. Some options for keeping them compact are nightly-only. Others have no rustfmt option. fastfmt runs rustfmt through stdin, then rejoins comment-free constructs within its size and width limits, giving fastai-style compact Rust from a stable toolchain.

Install and use

pip install fastfmt

The wheel contains one cargo-fastfmt binary and no Python code. With the binary on PATH, Cargo recognizes it as a subcommand:

cargo fastfmt            # format the current directory tree in place
cargo fastfmt --check    # exit 1 listing files that would change; write nothing
cargo fastfmt src lib.rs # format specific files or directories

rustfmt must also be on PATH. Install it with rustup component add rustfmt.

--width N sets the rustfmt line cap, which defaults to 160. Joined one-liners have tighter caps: 105 columns, or 80 for a two-statement block.

On a normal run, fastfmt creates or updates the target project's rustfmt.toml with disable_all_formatting = true. This prevents cargo fmt or editor rustfmt runs from undoing the compact formatting. fastfmt overrides the setting for its own pass. No pre-existing config file is needed.

--check never writes the config. It reports a missing or disabled guard as a required update.

Joining rules

A construct can join onto one line when it fits the width cap and contains no comments or multiline tokens. The supported constructs are:

• fn, if/else, and loop bodies with one statement or expression.
• match, struct, and enum bodies with up to three arms, fields, or variants.
• Single-item impl blocks.

Semicolons are preserved exactly. A statement-position else starts on a new line. An if/else used as a value stays on one line.

Joins run innermost-first and repeat until nothing more can join. This allows nested constructs to collapse fully. Blocks used as match-arm bodies stay expanded. Blocks containing comments or multiline strings retain rustfmt's output.